The present invention relates to a turbine generator and, in particular, relates to a turbine generator which is suitable for starting the turbine generator as a variable speed synchronous motor through connecting armature windings of the turbine generator with a thyristor type starting device during starting thereof.
These days, a combined cycle electric power generation has drawn attention in view of energy resorces saving.
FIG. 2 shows a constitution diagram of a combined cycle electric power generation system.
A combined cycle electric power generation unit consists of a gas turbine 10, a stream turbine 11 which uses remaining heat in the exhaust gas from the gas turbine 10 as heat source for the boiler and a turbine generator 12. A set of the gas turbine 10, the steam turbine 11 and the turbine generator 12 is arranged along a common axis, and since the capacity of a single turbine generator is limited, a unit plant is composed of in a multi axis form. In other words, a combined cycle electric power generation plant is located near urban area having a large electric power demand and is operated on DSS (every day start and stop) base, in that gas turbines 10 are successively started dependent upon the loads for the plant.
Further, because of combustion temperature increase for achieving a high efficiency a capacity of the gas turbine in the combined cycle electric power generation plant increases, and accordingly a capacity increase of a starting device for the gas turbine is necessitated.
In a conventional starting device for starting the gas turbine, a torque converter system was used in which a torque converter and an induction motor is directly coupled at a power train axial end.
However, because of manufacturable size limit of the torque converter, the capacity increase of the starting device became difficult, therefore realization of thyristor type starting system is strongly demanded. The thyristor type starting system is one in which the turbine generator 12 is started in a form of a variable speed synchronous motor by making use of a thyristor type starting device 13 and through provision of a change-over device 1 between the turbine generators 12 and the thyristor starting device 13, the single thyristor starting device 13 can be commonly used. Namely, the thyristor starting system can be used as a starting device for a particular gas turbine 10 having a large capacity as well as the single thyristor starting device 13 can be commonly used for starting other gas turbines 10 on multiple axises, therefore a compact building can accommodate all of these constituting devices.
On one hand, in a conventional generator, the rotor thereof is provided with field windings which excite the generator while receiving a DC current from an exciting current source and the stator thereof is provided with armature windings from which an electrical power is output.
In such generator, when an unbalanced load condition is caused, in which load conditions in the three phase armature windings are different with respective phases, a negative phase rotating magnetic field is generated in the rotor due to the unbalanced armature current.
The negative phase rotating magnetic field contains an asynchronous magnetic field component composed of a single component having an angular frequency 2.omega. which is two times of the angular frequency .omega. of the output voltage. When such negative phase rotating magnetic field is generated, an eddy current is induced in respective conductor portions on the rotor and temperatures at the respective portions on the rotor rise beyond a predetermined allowable range.
Accordingly, for resolving the above problem JP-B-60-34340(1985) proposes a counter measure in which current flowing in axial direction on the solid iron core rotor is led toward damper windings via conductive wedges and thereby a thermal balance at respective portions on the rotor is achieved. Further, JP-B-5-64015(1993) proposes another counter measure in which resistance values of damper bars inserted into solts on the rotor are varied in an alternate manner between large and small values in circumferential direction and thereby a thermal balance at respective portions on the rotor is achieved.
FIG. 3 shows a turbine generator system constitution diagram when the turbine generator is started by a thyristor type starting device, FIGS. 4(a) and 4(b) show armature current fed from the thyristor type starting device, FIG. 5 shows a relationship of magnetomotive force vectors induced in the machine during starting by the thyristor type starting device and FIG. 6 shows a magnetomotive force of sixth order higher harmonic wave induced during starting by the thyristor type starting device.
In the these drawings, numeral 14 denotes an exciter, CONV a converter, INV an inverter, numeral 2 a rotor of the turbine generator 12, numeral 3 slots provided on the rotor 2 for accommodating field windings.
In the thyristor type starting system, since the turbine generator 12 is started as a variable speed synchronous motor, it is necessary to form a rotating magnetic field of variable frequency at the armature side. Accordingly, during starting with thyristors the thyristor type starting device 13 constituted by the inverter INV and the converter CONV is connected for feeding a balanced three phase AC current of variable frequency to the armature winding thereof, and the field windings thereof are excited by DC current. The variable frequency of the balanced three phase AC current from the thyristor type starting device 13 is adjusted to one which is synchronous with the rotating frequency of the rotor 2 and is designed to increase dependent upon increase of the rotating speed.
The current fed to the armature windings from the thyristor type starting device 13 in such instance is in a distorted waveform as shown in FIG. 4(a). As illustrated in the current waveforms, in order to maintain a condition wherein two phases among the three phases are always in conductive state, the respective phases (U phase, V phase and W phase) likely repeat in a same manner a positive polarity conductive state of 120.degree. electrical angle, a rest state of 60.degree. electrical angle, a negative polarity conductive state of 120.degree. electrical angle and a rest state of 60.degree. electrical angle.
Spectrum of the current waveform of 120.degree. electrical angle conduction is shown in FIG. 4(b) wherein other than the current component of fundamental waveform higher harmonic current components of (6 m.+-.1)th order are contained in the current fed to the armature windings of the turbine generator 12 from the thyristor type starting device 13. Further, during starting with thyristors the magnetomotive force Fa of armature fundamental wave is designed to be perpendicular with respect to the magnetomotive force Ff of field windings in order to maximize torque to be generated as illustrated in FIG. 5. In this instance, since higher harmonic components of (6 m.+-.1)th order are contained in the armature current as explained above, magnetomotive force of high harmonics of 6 mth order are generated in the machine when looking at from the rotor side and elliptic rotating magnetic fields having the longer diameter on d axis and the shorter diameter on q axis are formed.
The formation of the elliptic rotating magnetic fields is explained with reference to FIG. 6 wherein an example when m=1, in that, the magnetomotive force of higher harmonic of 6th order is taken up. Although the higher harmonics components of 5th and 7th orders in the armature current form magnetomotive forces of 5th and 7th order higher harmonics when looking at from the stator side, the magnetomotive force of 5th order higher harmonic rotates in opposite direction with respect to the rotating direction of the rotor 2 to thereby constitute magnetomotive force of 6th order higher harmonic when looking at the rotor 2 and the magnetomotive force of 7th order higher harmonic rotates in the same direction with the rotating direction of the rotor 2 to thereby constitute magnetomotive force of 6th order higher harmonic when looking at from the rotor 2.
As will be understood from the above, the magnetomotive force of 6th order higher harmonic is consisted of two vector components and the two vector components show the same direction on d axis, therefore the resultant vector on d axis is the sum of the two vector components, however since these two vector components rotate in opposite directions each other, these two vector components show opposite directions on q axis, therefore the resultant vector is the difference between the two vector components. Thus, according to the sum of these two vector components shows an elliptic form having the longer diameter on d axis and the shorter diameter on q axis as illustrated in FIG. 6. The ratio of d and q axis components of the 6th order higher harmonic is usually at 6m:1.
Now, when an asynchronous magnetic field is generated, an eddy current is induced over the surface of the rotor so as to suppress the asynchronous magnetic field. Generators now in use are provided with damper bars arranged uniformely over the rotor without regarding the position of the magnetic poles in order to optimumly suppress an asynchronous magnetic field consisting of a single component such as a negative phase rotating magnetic field which is generated during unbalanced loading. However, the inventors found out that in case of an asynchronous magnetic field consisting of a plurality of components such as experienced during starting with thyristors a larger eddy current is induced on the rotor portions between the magnetic poles in comparison with one on the rotor portion at the magnetic poles, the resistance loss distribution over the rotor surface in circumferential direction becomes non-uniform and thus an optimum damper effect can not be achieved.